The present invention relates to circuitry for use with capacitive touch pads, such as are widely used in control panels for microwave ranges and for other appliances and, more particularly, relates to circuitry for use with touch pads associated with displays using multiplexed gas discharge or vacuum display tubes.
In an appliance utilizing both capacitive touch pads and multiplexed display tubes, it is necessary to scan both the pads and tubes at frequent intervals. It is highly desirable, from the viewpoint of economy in size and cost, that as much of the scanning circuitry as possible be used in common with both the touch pads and the display tubes. If this is done, however, certain problems of interference between touch pad circuitry and tube circuitry may occur. For example, if an anode of a display tube receives, by pickup and the like, a touch pad scan voltage pulse, the pulse will be coupled via the tube to the touch detection circuitry associated with the touch pad panel, creating significant imbalance in the touch pad response.
A second problem, arising with the use of capacitive touch pads, is a result of the frequent presence of contaminants (e.g., food) on the surface of the touch pad panel. The presence on the panel of materials, e.g. water, with its high dielectric constant (about 80 times that of air), or good ionic conductors like dishwashing detergent or salt, can cause coupling between adjacent touch pads, and present difficulty in making a determination of which of the pads has actually been touched. The conventional solution to this problem is to provide a relatively large spacing between adjacent touch pads. This solution is undesirable from the viewpoint of spatial considerations, such as economy and ease of fabrication.
A third problem is attributable to the requirements that the magnitude of the touch pad scan pulse voltage must be substantially constant while the magnitude of the display tube scan pulse voltage is generally varying. Particularly, the voltage magnitude of the display tube scan pulse (applied to the anode of a gas discharge tube) which is required to produce a predetermined tube brightness varies from tube to tube and varies for any given tube with time and temperature. Accordingly, the control circuitry generating the display tube scan voltage pulses must be capable of varying the voltage magnitude of these pulses to meet the particular requirements of each tube. In contrast, the voltage magnitude of the drive pulses applied to the touch pads must be maintained at a substantially constant level if a meaningful interpretation of the touch pad responses is to be made.
An additional problem is that when a touch pad is touched, power-line frequency, e.g. 60 cycle, energy is sometimes picked up and coupled into the touch pad, making it difficult for the circuitry to determine whether or not the pad has been touched.
One further problem that can arise, if adjacent touch pads are too close together, is that more than one pad can be simultaneously touched by the finger of the operator of the device being controlled by the touch pads, resulting in a spurious detection of a "touch" condition. It is desirable to solve this problem without increasing the spacing between adjacent touch pads.